(a) Field of the Invention
The present invention relates to nickel fine powder comprising nickel and a trace amount of phosphorus dispersed in the nickel as well as a method for preparing the same and more particularly to nickel fine powder which comprises nickel and a trace amount of phosphorus dispersed in the nickel, whose particle size is uniform and which is excellent in resistance to heat shrinkage as well as a method for preparing the same.
(b) Description of the Prior Art
Heretofore, there have been proposed a variety of methods for the preparation of nickel fine powder. These methods can roughly divided into two categories, one of which is a dry method comprising the steps of using a nickel salt such as nickel chloride as a starting material, subjecting the nickel salt to vapor-phase reduction with hydrogen and then cooling the resulting nickel vapor to give nickel fine powder; and the other of which is a wet method comprising the steps of mixing an aqueous solution containing a nickel salt such as nickel chloride with an aqueous solution of an alkali hydroxide to form nickel hydroxide and then reducing the resulting nickel hydroxide to give nickel fine powder.
Moreover, regarding methods for controlling the particle size of nickel fine powder, there have been known, for instance, those comprising adjusting the nickel salt vapor concentration and/or the amount of hydrogen gas to be introduced during the reduction step for the dry preparation method; and those comprising controlling, for instance, the concentration of each reaction component, the reaction temperature and the kinds and concentrations of additives, for the wet preparation method. However, there can be prepared simply nickel fine powder whose average particle size falls within the range of from 0.2 to 0.5 .mu.m, even if nickel fine powder having a small particle size is prepared according to either of the foregoing preparation methods.
It is difficult to prepare nickel fine powder having a average particle size smaller than that defined above according to the presently existing technology. In particular, there has never been proposed any technique for the steady preparation of nickel fine powder whose particle size is uniform and is less than 0.2 .mu.m.